1. Field of the Invention
The present invention relates to a hatchback door structure for vehicles, which is light and rigid.
2. Description of the Background Art
A hatchback door structure has been known, which includes an inner panel in which a lateral beam is integrally formed with a frame which has a shape outlining an outer shape of the hatchback door so that the lateral beam divides the frame into an upper portion and a lower portion (for example, refer to paragraphs 0010 to 0015 and FIG. 1 of Japanese Patent Laid-Open Publication Kokai No. H08-258568). Here, FIG. 14 is a schematic illustration of a vehicle seen from a backside for explaining a conventional hatchback door structure. As shown in FIG. 14, a hatch back door 201 of a vehicle V includes an inner panel 206 comprising a frame 209 and a lateral beam 218. A window glass 207 is fixed in an opening 212a zoned by the frame 209 and the lateral beam 218 on an upper side of the lateral beam 218. In an opening 212b zoned by the frame 209 and the lateral beam 218 on a lower side of the lateral beam 218, a pair of joint reinforcements 200a is disposed. A lower end of each reinforcement 200a is welded on the frame 209 through a lock reinforcement 200b which is fixed at a center of a bottom side of the frame 209. The pair of the joint reinforcements 200a extends in a V-shape to each upper corner of the opening 212b. An upper end of the joint reinforcement 200a is welded on the frame 209 and the lateral beam 218 through a corresponding corner reinforcement 200c which is fixed on a corresponding upper corner of the opening 212b. 
In the hatchback door structure 201, a rigidity of the inner panel 206 is increased by disposing the pair of joint reinforcements 200a in the opening 212b. 
Generally, a hatchback door fitted up on a tailgate in a rear part of a vehicle is jointed to the vehicle with a hinge structure. An opening-closing motion of the hatchback door is performed by rotating the door about the hinge located at the upper end of the door. These types of hatchback doors are required to increase the rigidity for improving collision strength and for avoiding muffled sounds when a vehicle runs a punishing road. On the other hand, the hatchback door must be light for reducing fuel consumption of a vehicle and improving easiness of the opening-closing motion of the door. Because of the above reasons, most of conventional hatchback doors for vehicles have a structure combining a thin outer panel and a thin inner panel which has openings, and the structure is reinforced by utilizing shapes of the both panels.
FIG. 15 is a perspective view showing an example of an inner panel which configures a conventional hatchback door structure. In an inner panel 302c of the example, a frame 304c which outlines an outer shape of the hatchback door is formed. In an upper portion of the frame 304c, a window frame 305c is formed and an opening 308c in which a window glass is to be set is disposed. An opening 309c is disposed in a lower portion of the inner panel 302c for weight saving. In the inner panel 302c, a lateral beam 306c which separates the two openings 308c, 309c is formed. The lateral beam 306c is integrally formed with the frame 304c by press-forming. The lateral beam 306c is a component for increasing a rigidity of the inner panel 302c against a lateral bending. Examples disclosed in Japanese Patent No. 3013921 and Japanese Unexamined Patent Publication Kohyo No. 11-514613 have identical contents with that of the aforementioned example, in which openings of the inner panel disposed for weight saving are reinforced by a beam or reinforcement and the like.
However, in the hatchback door structure 201 in FIG. 14, the discrete joint reinforcements 200a are welded on the frame 209 and the lateral beam 218, while requiring other members such as the lock reinforcement 200b and the corner reinforcements 200c for welding the joint reinforcements 200a. Therefore, the manufacturing cost becomes high, and at the same time, a weight of the hatchback door structure 201 increases. Further, a working process increases due to necessity of the welding.
In addition, in FIG. 15, since the lateral beam 306c for reinforcing the inner panel 302c, which is used for a conventional hatchback door structure, is integrally formed with the frame 304c when the inner panel 302c is formed by press-forming, there are following problems.
The first problem is as follows. When the lateral beam 306c is integrally formed as in the inner panel 302c shown in FIG. 15, a formation of cut-and-turned-up walls (hereinafter, referred to as louvered-walls) 311c with a sufficient height formed on peripheries of the openings 308c, 309c of the inner panel 302c is difficult due to a limitation in press formability, unless a highly-ductile material is used for the panel. Therefore, an increase of the rigidity of the inner panel 302c is limited.
The second is as follows. Corners 312c, where the lateral beam 306c and the frame 304c of the inner panel 302c are jointed, are required to have a radius R larger than a predetermined value. As a result, a size of the opening 308c for the rear glass window should be made smaller, which narrows a rearward visibility of a vehicle.
The third is as follows. Improvement of the rigidity of the inner panel 302c against bending in a tangential direction relative to a circular path formed by the rotation circle of the hatchback door is difficult. When a hatchback door is closed, the hatchback door receives a force in the tangential direction. Therefore, if the louvered-walls 311c of the lateral beam 306c are formed in parallel with the tangential direction, the vibrations of the hatchback door are effectively suppressed. However, since an inclination of the louvered-walls 311c of the lateral beam 306c is determined by a direction of the press-forming of the inner panel 302c, the louvered-walls 311c may not be inclined in the effective direction for suppressing the vibrations of the hatchback door when the door is closed.
The fourth is as follows. Since the lateral beam 306c is integrally formed with the inner panel 302c, a panel thickness of the lateral beam 306c can not easily be made different from that of the inner panel 302c. For example, changes in design to thin the panel thickness of the lateral beam 306c for the weight saving thinner than that of the frame 304c can not be performed.
It is, therefore, desired to provide a hatchback door structure for vehicles, which is light and capable of increasing a rigidity of the hatchback door structure, as well as capable of reducing a manufacturing process and cost.
It is also desired to provide a hatchback door structure for vehicles, which can improve a rearward visibility of a vehicle.